Mollicutes are surface parasites, adhering to eukaryotic cells via specific adhesins embedded in the plasma membrane. Their lack of rigid cell walls renders the Mollicutes able to enter into especially intimate relationships with host tissues. For Mycoplasma species, these are often mucosal surfaces of the respiratory and urogenital tracts of animals, including humans. Many, but not all, cause disease confined to these sites, while some are more highly invasive, causing systemic infections that often include arthritis as a primary or secondary manifestation. Mycoplasma arthritidis is such a species, causing an acute, self-limiting polyarthritis of rats and, under experimental conditions, a more chronic condition in mice and rabbits. These diseases have been used for many years as models to study various infectious and immunologic aspects of joint damage, although mechanisms of invasion of this and other arthritogenic mycoplasmas are poorly understood. However, the first step in this and most other infectious processes is attachment to host surfaces, and understanding this initial event is critical to eventual understanding of the events that follow. Cytadherence has been extensively characterized for two human respiratory and urogenital mycoplasmal pathogens, but similar work on the arthritogenic mycoplasmas is in its infancy. Two putative protein adhesins on the surface of virulent M. arthritidis strain 158p10p9 have been identified with attachment- inhibiting monoclonal antibodies, and a low-adherence variant missing one of these proteins has been identified; it actually carries what may be a truncated, nonfunctional version of the "missing" adhesin. Not all strains of M. arthritidis express both adhesins, and in some, they seem to undergo size variation. Further characterization of these putative adhesins is proposed, including confirmation of surface orientation, determination of lipid modification, and peptide mapping. Genes for these proteins will be cloned and sequenced and used as probes to identify related genes in M. arthritidis strains not expressing one or both proteins or expressing different size variants. This analysis will also include the low-adherence mutant carrying an apparently truncated version of one of them. Adhesin sequences will also be compared with those published for other Mollicutes and with other sequences found in GenBank. For further characterization insertion mutations will be created in genes coding for the two putative adhesins already identified, and random mutations will be introduced into the chromosome to identify additional surface proteins associated with attachment. Such, proteins will also be sought by use of attachment- inhibiting monoclonal antibodies. Finally, putative adhesins will be used to immunize rats against M. arthritidis-induced arthritis and adherence- negative mutants tested for ability to induce arthritis in order to confirm a role in virulence. Elucidation of the mechanisms by which this pathogen interacts with cells of its host should help identify ways in which other arthritogenic mycoplasmas of humans and animals begin the process that eventually leads to joint damage and destruction.